How to Construct an Astrolabe, Using Your PC

by Christine Craig

There is a tale, both apocryphal and scatological, that Claudius Ptolemy, the Alexandrian astronomer (ca. 90 A.D.- 168 A.D.) whose ideas dominated astronomical thought for well over a millennium (Figure 1), got the idea for the planispheric astrolabe while riding a donkey. The armillary sphere he was carrying fell and was flattened by the donkey’s hoof into a pile of fresh donkey dung. Upon inspecting the resulting impression, a candle ignited in his mind, leading to the creation of an astronomical instrument so useful, that it outlasted Ptolemaic astronomy itself. Because the first preserved astrolabes are made of brass and dated since the time of Muhammad, and the first known treatise on the astrolabe was written well before Muhammad, it is unknown when and where the astrolabe was born— surely not full-grown and fully adorned, like Athena from the head of Zeus. Early astrolabes probably long predated the technology for accurately rendering the requisite lines and arcs onto brass. Paper, cloth, and wood were more likely the media for the first astrolabes.